1. Field of the Invention
This invention relates to in-line microwave warming apparatus for warming blood and other fluids.
2. Background Information
In many applications, particularly in the medical field, there may be a requirement that a circulating fluid be warmed. For example, in cardiac surgery during extra-corporeal circulation (ECC), the patient is first cooled in order to slow metabolism and thereafter the circulating blood is warmed to return it to body temperature. As another example, heated intravenous fluids are useful in hypothermic patients and in trauma patients requiring massive IV resuscitation.
Microwave energy has, in the past, been used in connection with the heating of blood and intravenous fluids. For example, my U.S. Pat. Nos. 5,073,167 and 5,919,218, whose entire contents are incorporated herein by reference, disclose microwave apparatus comprising a waveguide heating cavity having a source of microwave energy coupled thereto. A support element encircled by a fixed length of tubing forms a disposable cartridge which may be positioned in the heating cavity. The characteristics, and placement within the heating cavity, of the cartridge are such that there results a rapid, uniform heating of the fluid flowing through the cartridge.
Such prior apparatus also includes means for non-invasively monitoring the temperature of fluid flowing through the cartridge and thereby controlling the energy source so as to maintain the flowing fluid at a selected temperature. These means include an external fluid inlet temperature transducer and an external fluid outlet temperature transducer. Since these transducers are external to the heating cavity, a third transducer is needed to measure the temperature of the fluid within the cavity. This is necessary to address the situation wherein the fluid flow is suddenly stopped for some reason and the output transducer is calling for heat because it senses a temperature drop. In other words, the fluid could severely overheat before the outlet transducer recognizes the problem. Resultantly, when flow resumes, the overheated fluid could injure the patient.
While the above-described patented in-line microwave warmers provide distinct advantages over the prior water immersion-type warmers, they have certain drawbacks which may limit their use and application. For example, as noted above, they require three separate temperature monitors each of which consists of a transducer and a radiometer. Also, the cartridges in the patented apparatus require multiple turns of tubing in order to achieve the desired warming effect. Such a multi-turn tubing cartridge is quite large and has a relatively large priming volume, in the order or 4 ml. In addition, the large cartridge necessitates a commensurately large opening in the heating cavity in order to receive the cartridge. This means that steps must be taken to ensure that microwave radiation does not leak from the heating cavity at that opening. For example, the patented cartridge is provided with a complex metal ground plane to inhibit such radiation leakage.
Still further, in the prior apparatus reflected in the above patents, the transducer in the heating cavity receives signals from all of the windings in the cartridge and accordingly senses the average temperature of the fluid in the multiple windings rather than the temperature of the fluid just as the fluid exits the heating cavity.
Still further, the two external transducers, three separate external radiometers and the multiple cables connecting the various temperature transducers to the radiometers, increase the overall complexity and footprint of the prior apparatus.